Standalone thermal energy recycling device for engine after-treatment systems

ABSTRACT

A standalone thermal energy recycling device in which exhaust air emitted from an internal combustion engine exchanges heat with that being heated during regeneration of after-treatment devices. By using the thermal energy recycling device, heat generated for regeneration is used to compensate energy loss during heat exchange rather than provide the overall energy needed for boosting exhaust flow temperature to target. Inside the thermal energy recycling device, heat exchanger is bypassed during normal engine operations for decreasing back pressure.

This present application claims priority from U.S. provisionalapplication No. 60/850,459 having the title of Engine AftertreatmentSystem with Thermal Energy Recycling and filed on Oct. 10, 2006.

FIELD OF THE INVENTION

The present invention relates to devices for effectively boosting andcontrolling temperature of exhaust gases emitted from an internalcombustion engine to facilitate regeneration of after-treatment systems.

BACKGROUND OF THE INVENTION

Reducing oxides of nitrogen (NOx), carbon monoxide (CO), hyrdrocarbon(HC), and particulate matter (PM) in exhaust air from internalcombustion engines is required in emission control. DOCs (DieselOxidation Catalyst) and three-way catalysts have been broadly used forreducing CO, HC, and NOx, while SCR (Selective Catalytic Reduction), LNT(Lean NOx Trap, a.k.a. NOx adsorber), LNC (Lean NOx Catalyst), and EGR(Exhaust Gas Recirculation) technology are used for achieving low NOxemissions. The PM in exhaust air usually is removed by using a filter,e.g. DPF (Diesel Particulate Filter).

PM filters and LNTs need to be regenerated periodically. For PM filters,especially DPFs, since normal engine out temperature is not high enoughto oxidize soot automatically, a number of known methods are used forthe regeneration of particulate filters. In addition to passiveregeneration methods at low exhaust temperature (˜300° C.), such asusing fuel additives and using NOx generated in the engine, the mostcommon one is thermal regeneration, i.e., heating the exhaust gas to atemperature higher than 500° C. (usually 500˜600° C.) and thenconverting soot matter into gaseous products such as CO₂ and H₂O. InLNTs, injecting fuel (HC reductant) into rich exhaust is used forreducing metal nitrate into metal oxide, and heating the exhaust gas toa high temperature (normally higher than 650° C.) together with richfuel dosing for reducing metal sulfate.

Electrically resistive heating devices, fuel burners, and oxidationcatalyst converters have been used for generating heat in thermalregeneration. However, due to high exhaust flow rate, most ofregeneration energy is actually used for increasing enthalpy of exhaustair instead of providing enough temperature for soot to be oxidized orfor reducing metal sulfate. It usually needs 10 g to 40 g fuel to burnoff 1 g soot and the regeneration power usually is 15 kw to 80 kwdepending on exhaust flow rate and temperature.

Due to the large power needed in regeneration, resistive heating devicesnormally cannot be directly used for mobile applications without exhaustflow control, while fuel burners are complex and it is hard to controlthe regeneration temperature. DOCs are simple and can generate enoughheat for regeneration. However, a minimum light-off temperature isrequired for oxidation reaction. As a result, it is difficult to useDOCs for engines with low exhaust temperature, e.g. engines withtwo-stage turbochargers. And in transient applications, e.g. vehiclesstop and go frequently, when turbo outlet temperature frequently dropsbelow light-off temperature, regeneration cannot be effectivelyperformed.

It is a primary object of the present invention to reuse the energy forboosting exhaust temperature in regeneration. With the reuse ofregeneration energy, in addition to better fuel economy, the powerneeded for regeneration is decreased, and more heating devices can beused to facilitate temperature control.

Another object of the present invention is to facilitate the temperaturecontrol for after-treatment systems with DOCs and enable the systemswork with low turbo outlet temperature and/or under transient dutycycles.

Yet another object of the present invention is to develop a standalonedevice that is independent to the after-treatment system and thus can beused with different types of after-treatment systems.

Yet another object of the present invention is to develop a device thatcan be bypassed during normal operations when the after-treatment systemis not in regeneration, therefore, the device has minimum effects onengine backpressure.

Yet another object of the present invention is to decreaseafter-treatment system cost and elongate system lifetime.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a thermal energy recycling device in aninternal combustion engine after-treatment system. In this device, hightemperature exhaust air heated for regenerating after-treatment devices,such as DPFs or LNTs, is cycled back and exchanges heat with lowtemperature exhaust air at inlet. After heat exchange, the heatedexhaust air then enters after-treatment system for regeneration, and thecooled exhaust air is emitted into ambient or flows into otherafter-treatment devices for further treatment.

In an exemplary embodiment of the present invention, the after-treatmentsystem comprises a DOC, a DPF, and a heat exchange device with two portsconnected with the outlet of a turbocharger and the inlet of the DOC,and another two connected to the DPF outlet and an exhaust pipe. Duringregeneration, the exhaust temperature is boosted by oxidizing fuel inthe DOC. The high temperatures exhaust air then enter DPF forregeneration. From the downstream of the DPF, the high temperatureexhaust air flows back to the heat exchange device where it loses heatto the low temperature exhaust air entering the device from the turbooutlet. The heated exhaust air then flows into the DOC where the newdosing fuel is injected for compensating the heat loss in DOC, DPF, andthe heat exchange device. Heat exchange is only used for facilitatingafter-treatment system regeneration. During normal operations, todecrease engine back pressure, the heat exchange device can becontrolled bypassing exhaust air to the after-treatment system.

In accordance to an advantageous feature of the invention, good fueleconomy is achieved by burning less fuel in the DOC due to the exhaustflow at DOC inlet has a higher temperature.

In accordance with another advantageous feature of the invention, DOCface plugging risks are mitigated. When exhaust air temperature at DOCinlet is low and dosing time is long, dosing fuel could mix with sootand form a layer at DOC front face blocking exhaust air from passingthrough. This is called face plugging. High DOC inlet air temperaturegained from high DPF outlet temperature reduces the risk of faceplugging.

In accordance with another advantageous feature of the invention, DOCsize can be decreased, since less dosing and higher DOC inlettemperature enables the after-treatment system use a DOC with lower HCconversion efficiency.

In accordance with another advantageous feature of the invention, theafter-treatment system can work with a low exhaust temperature and isinsensitive to the variation of exhaust temperature. Once the DOC isable to generate enough heat to meet the target temperature,regeneration is un-interrupted as long as the energy released by burningdosing fuel can compensate the heat loss in DOC, DPF, and the heatexchange device. The heat loss is determined by DOC and DPF size,insulation, and heat exchange efficiency and is not much affected by theturbo outlet exhaust temperature. This feature is especially useful forengines with low exhaust temperature (e.g. engines with two-stageturbocharger). With this feature, regeneration can be started bymomentarily creating an exhaust flow with temperature higher than DOClight-off temperature (e.g. by adjusting turbo and EGR, or using anelectric heater). Once the DOC is able to generate enough heat tosustain target temperature, the engine can run at its normal mode withlow exhaust temperature.

In accordance with another advantageous feature of the invention, due toa positive feedback, the temperature rising time is shortened duringdosing. This feature facilitates temperature control.

In accordance with another advantageous feature of the invention, whenan external doser is used for injecting fuel, less dosing fuel elongatedoser lifetime. Furthermore, since the heat exchange device is astandalone device, the external doser is able to be placed in betweenthe heat exchange device and the DOC. As a result, dosing fuel needs notpass the heat exchange device causing plugging issues and dosing fuelimpingement issue is mitigated due to higher exhaust temperature.

In accordance with another advantageous feature of the invention, whenregeneration starts, the heat exchange device switches from bypassingmode to heat exchange mode, then a higher engine back pressure isinduced. This higher engine back pressure increases engine out exhausttemperature, and thereby facilities regeneration.

In accordance with a further advantageous feature of the invention,safer low exhaust air is emitted to ambient during regeneration due toheat exchange.

In another embodiment of the present invention, the after-treatmentsystem comprises a DOC, a DPF, a SCR (or an LNT), and a heat exchangedevice with two ports connected between the outlet of a turbocharger andthe inlet of the DOC, and another two connected to the DPF outlet andthe SCR (or the LNT) inlet. The SCR (or the LNT) is at the very end ofthe after-treatment system.

In accordance with an advantageous feature of the invention, the heatexchange device intrinsically protects the SCR (or the LNT) from beingdamaged by thermal runaways in the DPF.

Yet in another embodiment of the present invention, the after-treatmentsystem comprises a DOC, a DPF, an LNT, and a heat exchange device, inwhich high temperature exhaust air emit from the LNT during desulfationexchanges heat with low temperature exhaust air. In accordance with anadvantageous feature of the invention, during desulfation, thetemperature dropping is minimized in rich cycle due to the energyexchange between the exchange device and exhaust air, and temperaturerising time is shortened in lean cycle. Thereby, desulfation process ismore effective.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a illustrates an after-treatment system according to the presentinvention including a DOC, a DPF, an external doser, and a heat exchangedevice;

FIG. 1 b illustrates an after-treatment system according to the presentinvention including a DOC, a DPF, an external doser, and a heat-pumplike heat exchange device;

FIG. 1 c shows a heat exchange device with exhaust air bypassing;

FIG. 2 shows an after-treatment system with in-cylinder dosing (postinjection) according to the present invention including a DOC, a DPF,and a heat exchange device;

FIG. 3 depicts an after-treatment system according to the presentinvention including a fuel burner, a DPF, and a heat exchange device;

FIG. 4 shows an after-treatment system according to the presentinvention including a control valve, a blower, a resistive heatingdevice, a DPF, and a heat exchange device;

FIG. 5 a shows a SCR device installed at the very end of anafter-treatment system according to the present invention;

FIG. 5 b shows a LNT device installed at the very end of anafter-treatment system according to the present invention;

FIG. 6 a illustrates an after-treatment system according to the presentinvention including a DPF, an LNT, a DOC, and a heat exchange device;the DOC and DPF connect to the heat exchange device;

FIG. 6 b shows an after-treatment system according to the presentinvention including a DOC, an LNT, a DPF, and a heat exchange device;the DOC, DPF and LNT all connect to the heat exchange device, and theLNT is in front of the DOC;

FIG. 6 c shows an after-treatment system according to the presentinvention including a DOC, an LNT, a DPF, and a heat exchange device;the DOC, DPF and LNT all connect to the heat exchange device, and theDOC is in front of the LNT.

DETAILED DESCRIPTION OF THE INVENTION

As depicted in FIG. 1, in an after-treatment system, a standalone heatexchange device 103 is connected with a turbocharger 101 through a pipe102. Another pipe 104 is used to connect the heat exchange device 103 toa DOC 108, in which the HC, CO, and NOx are oxidized. Particulate matter(PM) generated by the engine is trapped in a DPF 110. A pipe 105connects the DPF 110 back to the heat exchange device 103, and a pipe113 conducts exhaust air off the after-treatment system.

When certain amount of PM deposits in the DPF 110, a regenerationprocess is triggered, and HC injected from a doser 106 is oxidized inthe DOC 108 to provide heat for burning off PM in the DPF 110. Duringthe regeneration process, the DOC inlet temperature is measured by athermistor 107, while the DOC outlet temperature is monitored by using athermistor 109. To effectively monitor the DOC conversion efficiency anddetect thermal runaways inside the DPF, a thermistor 1111 is connectedto the outlet of the DPF 110. A relative pressure (deltaP) sensor 112 isused to monitor the DPF pressure drop, which changes with the PM amountin the DPF. In normal after-treatment systems, during regeneration, thehigh temperature exhaust air from DPF is emitted into ambient airdirectly, and most of the energy released by oxidizing HC in DOC iswasted. In the present invention, however, the exhaust air from the DPF110 is conducted back to the heat exchange device 103, in which the hightemperature air exchanges thermal energy with the low temperatureexhaust air emitted from the turbo 101. The heated exhaust air then goesinto the DOC 108, therein its temperature is further boosted forregeneration. In the DOC 108, the amount of energy released by oxidizingfuel is used to compensate the energy loss during the heat exchange, andthe heat dissipated in the DOC 109 and the DPF 110, rather than providethe overall energy needed for heating low temperature exhaust air fromengine to the target temperature. A variety of heat exchangers, forexample, shell and tube heat exchangers, plate heat exchangers, andenthalpy wheels, can be used for the heat exchange device 103.Additionally, a heat-pump like heat exchange device can also be used todeliver the heat generated in regeneration to DOC inlet. As depicted inFIG. 1 b, this device 150 includes a pump 151, a coil 152 at DOC inlet,a coil 153 at DPF outlet, and a connection tube 154 with insulation 155.During regeneration, high temperature exhaust air at DPF outlet heats upthe fluid in the coil 153. Then driven by the pump 151, the heated fluidmoves into the coil 152 and exchange heat there with low temperatureexhaust air.

The heat exchange device 103 may cause higher engine back pressure,especially when heat exchange efficiency is high. To decrease engineback pressure during normal operations, as shown in FIG. 1 c, inside theheat exchange device 103, two control valves 161 or 162 can be used tobypass a heat exchanger 160. When the valves 161 and 162 are on, exhaustair passes through connection pipes 163 and 164 rather than the heatexchanger 160. Thereby, the engine back pressure is decreased.

Better Fuel Economy

The after-treatment system presented in this invention facilitiesregeneration. Firstly, due to heat exchange, the dosing fuel is used tocompensate the energy loss in DOC, DPF, and heat exchange process,rather than provide the overall energy for sustaining regenerationtemperature. Therefore, fuel economy is improved.

Less DOC Face Plugging

When exhaust air temperature at DOC inlet is low and dosing time islong, dosing fuel could mix with soot and form a layer at DOC front faceblocking exhaust air from passing through. This is called face plugging.High DOC inlet air temperature gained from the high DPF outlettemperature reduces the risk of face plugging.

Smaller DOC Size and Less Hydrocarbon Breakthrough

At steady status, with a given exhaust mass flow rate m_(exh) ^(•) andDOC inlet exhaust temperature T₁, the hydrocarbon mass flow ratem_(fuel) ^(•) needed for exhaust air to reach a target temperature T_(t)can be estimated using the following equation:

m _(fuel) ^(•)=(T _(t) −T ₁)m _(exh) ^(•) C _(p)/(LHVη)  (1)

where LHV is the low heat value of the dosing fuel; C_(p) is the averageheat capacity at constant pressure, and η is the HC conversionefficiency of the DOC.

According to the equation (1), to obtain the same target DOC outlettemperature T_(t), with a higher DOC inlet temperature T₁, a lower DOCefficiency and thus a smaller size DOC is allowed. In addition, based onthe equation (1), the hydrocarbon breakthrough rate S_(HC) ^(•) at DOCoutlet is given by

S _(HC) ^(•) =m _(fuel) ^(•)(1−η)=(m _(exh) ^(•) C _(p)/(LHV)[(T _(t) −T₁)(1−η)/η]  (2)

This equation (2) shows that a higher DOC inlet temperature T₁ reducesthe hydrocarbon breakthrough at a given conversion efficiency. ShorterTemperature Response Time

When HC is oxidized in a DOC, since reactions take place at catalystsurface, DOC base and catalyst absorb released energy. As a result, whendosing starts, exhaust air temperature cannot increase immediately.There exists a time lag between fuel dosing and exhaust temperaturechange. For a given DOC, this time lag is a function of exhaust flowrate. When exhaust flow rate is low, it takes longer time for exhausttemperature to reach target.

With the heat exchange device, a positive feedback is established whendosing starts: exhaust air is heated in DOC and the hot air through DPFthen exchanges heat with the low temperature exhaust air in the heatexchange device. The higher temperature DOC inlet exhaust air is thenfurther heated in DOC. This positive feedback shortens the exhaust airtemperature rising time, and thus facilitates temperature control.

Insensitive to the Variation of DOC Inlet Temperature

Under some operating conditions, for example, running at low torque,diesel engine generates less exhaust heat, and exhaust air temperatureis low. When exhaust air temperature in DOC is lower than catalystlight-off temperature, fuel dosing has to be disabled, otherwise,un-burnt fuel could cause DOC face plugging and hydrocarbonbreakthrough. Dosing can only be started when DOC temperature is higherthan catalyst light-off temperature. However, limited to heat exchangerate, there is a time lag between fuel dosing and exhaust airtemperature change. If DOC inlet exhaust temperature drops belowlight-off temperature frequently, regeneration cannot be effectivelyperformed.

With the heat exchange device, heated by the exhaust air fed back fromthe DPF, the DOC inlet temperature can still sustains higher thancatalyst light-off temperature even when turbo outlet temperature islow. The high DOC inlet temperature then allows continuous dosing and,therefore, the DOC outlet temperature and DPF outlet temperature arehigh. Regeneration is un-interrupted as long as the energy released byburning dosing fuel is able to compensate the heat loss in DOC, DPF, andthe heat exchange device. This feature is especially useful for engineswith low exhaust temperature (e.g. engines with a two-stageturbocharger). With this feature, regeneration can be started bymomentarily creating an exhaust flow with temperature higher thancatalyst light-off temperature (e.g. by adjusting turbo and EGR, orusing an electric heater). Once the DOC is able to generate enough heatto sustain target temperature, the engine can run at its normal modewith low exhaust temperature.

Longer Doser Life

When an external low-pressure doser is used, normally the injector ofthe doser exposes to exhaust air. After dosing, the fuel that remains inthe injector and on the injector surface could be coked and then blockthe orifice of the injector. As a result, with the same duty cycle, lessand less fuel will be injected by the doser at normal fuel pressure.When the maximum achievable dosing rate is lower than that required forreaching regeneration target temperature, the doser needs to bereplaced, otherwise the filter cannot be effectively regenerated, andthe system may fail.

With the heat exchange device, due to the high DOC inlet temperature,less fueling is needed for reaching regeneration target temperature.Therefore, even the doser deteriorates, as long as it can provide enoughdosing fuel to compensate the heat loss in DOC, DPF, and during heatexchange, the system is able to sustain the target temperature forregeneration. Less dosing rate requirement elongates doser life time.

Less Impingement Induced Dosing Fuel Condensation

Limited to the size of the connection pipe at which a doser isinstalled, normally, when dosing fuel is sprayed out of the doser, someof the fuel droplets may hit the inner wall of the connection pipe. Ifthe inner wall temperature is low, these fuel droplets may condense atthe wall surface causing DOC face plugging and exhaust air temperaturecontrol issues (e.g., when fuel evaporates with higher temperatureexhaust flow, this extra fuel flows into DOC causing exhaust temperatureout of control).

The standalone heat exchange device allows doser be installed in betweenit and the DOC. High temperature exhaust flow emitted from the heatexchange device keeps connection pipe inner wall from being cooled downby ambient temperature. As a result, dosing fuel condensation ismitigated.

Safer Exhaust

High temperature exhaust air during regeneration could cause fire hazardif the regeneration is performed in an inappropriate place, e.g. theexhaust pipe is close to combustible matter. With the heat exchangedevice, temperature of the exhaust air off the after-treatment system islowered during heat exchange, and thus less effort is needed to lowerexhaust temperature during regeneration.

In addition to external dosing depicted in FIG. 1, the after-treatmentsystem in the present invention can also use in-cylinder dosing, whichis achieved by injecting fuel into some cylinders of an engine duringexpansion stage. As illustrated in FIG. 2, an engine 201 is connected toan after-treatment system similar to the one shown in FIG. 1 a exempt noexternal doser 106 is included. In this after-treatment system, energyfor DPF regeneration is provided by oxidizing unburnt HC from theengine.

FIG. 3 shows an after-treatment system using a fuel burner 301 insteadof the DOC 108 for DPF regeneration. In this system, the burner includesa blower 310, a fuel pump 311, and a glow plug 312. The blower 310provides airflow for burning the fuel injected with the pump 311. Thehot air generated in the burner 301 mixes with exhaust flow and theresult air flow enters the DPF 110 for regeneration. Exhaust air emittedfrom the DPF 110 flows through the heat exchange device 103, where lowtemperature exhaust flow is heated. Compared to an after-treatmentsystem without the heat exchange device 103, in the present system, thefuel burner 301 provides less energy in boosting the exhaust flowtemperature to regeneration target temperature.

If an electrical heating means is used, as illustrated in FIG. 4, theenergy for regeneration can also be recycled. In this system, anelectrical heater 401 is used instead of a DOC 108 for generating hotair. A valve control system 412 is used for controlling exhaust flowduring regeneration. During a stationary regeneration, when the engineis not capable to generate exhaust flow, a blower 410 provides airflowthrough a passage 411. The heat exchange realized in the device 103lowered the need for electrical power.

If a SCR (Selected Catalyst Reduction) device is connected to the outletof a DPF, a tighter temperature control is needed, since hightemperature generated in a thermal runaway, which is caused byuncontrolled burning of large amount of soot accumulated in the filter,could damage the catalyst in SCR. When the energy exchange device isused, as depicted in FIG. 5 a, where a SCR 500 is connected to theexhaust pipe 113, exhaust temperature at SCR inlet is decreased duringheat exchange, and thus the system intrinsically protects the SCR 500from being damaged. If DPF regeneration is well controlled or thermalrunaway is not an issue, the SCR can also be placed in somewhere betweenthe DOC 108 and the heat exchange device 103. In such a system, withfuel dosing, SCR inlet temperature can be controlled in an appropriaterange for deNOx reaction at small energy cost.

When an LNT (Lean NOx Trap) 600 is connected to the exhaust pipe 113(FIG. 5 b), as in the SCR after-treatment system shown in FIG. 5 a, theheat exchange device could protect the LNT from being damaged by thermalrunaways in the DPF. If the LNT 600 is connected directly to the DPF 110as illustrated in FIG. 6 a, however, the heat exchange device is notable to mitigate the effects of thermal runaways in DPF though all otherbenefits mentioned above can still be provided. In the configurationsillustrated in FIG. 6 b and FIG. 6 c, since the DPF 110 is at the veryend of the after-treatment system, thermal runaways in the DPF won'taffect the performance of the LNT.

In a system with LNT, in addition to filter regeneration, a desulfationprocess is needed to decompose the sulfate formed due to sulfur in fuel.Usually, high bed temperature (normally higher than 650° C.) and a richexhaust are needed for desulfation. However, in rich exhaust, due to lowoxygen concentration, hydrocarbon cannot be effectively oxidized incatalyst. As a result, the LNT bed temperature drops and desulfationefficiency decreases. When LNT bed temperature is lower than requireddesulfation temperature, a lean exhaust is needed to increase theexhaust temperature. With the heat exchange device 103 (FIG. 5 a, FIG. 6a-6 c), the temperature drop is decreased in energy exchange, andtemperature rising time is shortened during lean exhaust time. Thereby,the rich exhaust time can be elongated and desulfation process is moreeffective.

1. An internal combustion engine after-treatment system comprising: afilter that traps PM (Particulate Matter) in exhaust air; a heatgeneration device that generates heat for the regeneration of saidfilter; an energy exchange device in which exhaust air emitted from saidinternal combustion engine exchanges heat with that heated with saidheat generation device; said filter and said heat generation device areoutside said energy exchange device.
 2. A system as in claim 1, whereinsaid heat generation device includes a DOC (Diesel Oxidation Catalyst)device;
 3. A system as in claim 2, wherein said heat generation deviceincludes an external doser for fuel injection;
 4. A system as in claim2, wherein said external doser is placed in between said heat exchangedevice and said DOC;
 5. A system as in claim 2, wherein said heatgeneration device uses in-cylinder post fuel injection;
 6. A system asin claim 1, wherein said heat generation device includes a fuel burner;7. A system as in claim 6, wherein said fuel burner includes a fuel pumpand an ignition device;
 8. A system as in claim 1, wherein said heatgeneration device includes an electrical resistive heating device;
 9. Asystem as in claim 1, wherein said energy exchange device includes aheat exchanger through which two air flows exchange heat;
 10. A systemas in claim 9, wherein said energy exchange device includes an exhaustair bypass apparatus that is controlled to bypass said heat exchanger;11. A system as in claim 1, wherein said after-treatment system furtherincludes a SCR (Selective Catalytic Reduction) device;
 12. A system asin claim 1, wherein said after-treatment system further includes an LNT(Lean NOx Trap) device;
 13. A system as in claim 1, wherein saidafter-treatment system further includes an LNC (Lean NOx Catalyst)device;
 14. An internal combustion engine after-treatment systemcomprising: an LNT device; a heat generation device that generates heatfor the regeneration of said LNT device; an energy exchange device inwhich exhaust air emitted from said internal combustion engine exchangesheat with that heated with said heat generation device; said filter andsaid heat generation device are outside said energy exchange device. 15.A system as in claim 14, wherein said heat generation device includes aDOC;
 16. A system as in claim 15, wherein said heat generation deviceincludes an external doser for fuel injection;
 17. A system as in claim16, wherein said external doser is placed in between said heat exchangedevice and said DOC;
 18. A system as in claim 14, wherein said energyexchange device includes a heat exchanger through which two air flowsexchange heat;
 19. A system as in claim 18, wherein said energy exchangedevice includes an exhaust air bypass apparatus that is controlled tobypass said heat exchanger;
 20. A system as in claim 14, wherein saidafter-treatment system further includes a DPF;